A first conventional optical fiber gyro comprises an optical fiber sensing loop through which lights radiated from a laser light source propagate in the clockwise (CW) and counter clockwise (CCW) directions, a phase modulator for modulating the light to be propagated through the sensing loop, a light detector for receiving the light propagated through the sensing loop to generate an electric signal, and a signal processing circuit for processing the electric signal to calculate an angular velocity of a rotating member on which the sensing loop is mounted.
In operation, a light radiated from the laser light source is divided into lights to be propagated through the sensing loop in the CW and CCW directions. Then, the lights propagated through the sensing loop in the CW and CCW directions are coupled to be supplied to the light detector, from which an electric output signal corresponding to the received light is supplied to the signal processing circuit.
In the signal processing circuit, an angular velocity of the rotating member is detected in accordance with Sagnac phase difference .phi.s which is generated in the CW and CCW propagating lights by Sagnac effect. The Sagnac effect is described in, for instance, the U.S. Pat. No. 5,272,516, and the relation between the angular velocity .OMEGA. and the Sagnac phase difference .phi.s is defined below. EQU .OMEGA.=(1/a).phi.s (1)
where a is a constant.
The output signal supplied from the light detector to the signal processing circuit comprises a DC (direct current) component, a fundamental wave component, a duplicate harmonic wave component, a triplicate harmonic wave component, a quadruple harmonic wave component, etc. which are defined by output signals corresponding to the CW and CCW lights, triangular functions of the Sagnac phase difference and the phase modulating frequency, and respective orders of Bessel functions based on a phase modulating degree. The output signal is synchronously detected in the signal processing circuit to generate a fundamental wave component S.sub.1, a duplicate harmonic wave component S.sub.2, and a quadruple harmonic wave component S.sub.4.
In accordance with the synchronous detection of the output signal, the angular velocity is detected as explained below.
(1) When the phase modulation of the CW and CCW lights is carried out to make the ratio S.sub.4 /S.sub.2 constant, the ratio of the first and second order Bessel functions is constant. Thus, the angular velocity .OMEGA. is detected by using an Arctan (tan.sup.-1) function based on the detected ratio s.sub.1 /S.sub.2.
(2) When the phase modulation of the CW and CCW lights is not carried out to make the ratio S.sub.4 /S.sub.2 constant, the ratio of the first and second order Bessel functions is calculated in accordance with the ratio of the second and fourth order Bessel functions which is calculated from the non-constant ratio S.sub.4 /S.sub.2. Thus, the angular velocity .OMEGA. is detected by using an Arctan (tan.sup.-1) function based on the detected ratio S.sub.2 /S.sub.1 and the calculated ratio of the first and second order Bessel functions.
In general, the fundamental wave component S.sub.1 is multiplied in a pre-amplifier by an amplication factor K. Thus, a multiplied value S (=S.sub.1 .times.K) is obtained to enhance the detecting sensitivity of an angular velocity, and the multiplied value S is used to determine as to whether a measuring range should be changed over. In such a case, the value S/K is used as a fundamental wave component S.sub.1 to calculate an angular velocity .OMEGA..
Other than the above described conventional optical fiber gyro, the Japanese Patent Kokai No. 4-231814 which was laid open on Aug. 20, 1992 describes a second conventional optical fiber gyro which comprises a memory for storing a relation of output signals and angular velocities in the form of a polygonal line, and characteristic amounts of the polygonal line.
In operation, an angular velocity is detected by reading the angular velocity from the memory based on a detected output signal in consideration of the characteristic amounts.
In the first conventional optical fiber gyro, however, there are disadvantages in that a response speed is low, because using an Arctan (tan.sup.-1) function takes a long time, and detecting/precision is not high, because (1) a result of the Arctan (tan.sup.-1) function and a detected value do not coincide to each other in accordance with the non-linear characteristics of an optical system and an electric circuit system in the optical fiber gyro, (2) a measuring range is changed over by the value S (=S.sub.1 .times.K), thereby shifting a measuring range changing point in accordance with the change in output power of a light source, so that the state is changed between calibration and operation, (3) the divisional calculation S/K (=S.sub.1) is carried out, and (4) the detected angular velocity includes an error caused by the rotation of the earth.
The second conventional optical fiber gyro has a disadvantage in that detecting precision is not high, because a deviation amount between a value of an Arctan (tan.sup.-1) function and a detected value is different for each optical fiber gyro, in addition to the same disadvantages as discussed in the first conventional optical fiber gyro.